<?php
if (isset($_POST['form_id']) && $_POST['form_id'] === 'test') {
$query = "
SELECT
`col1`,
`col2`
FROM
`table`
WHERE
`category_id` = " . (int) $_POST['category'] . "
";
echo $query;
}
?><!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>New</title>
</head>
<body>
<form method="post" action="">
<p>Kategorie:
<select name="category">
<option value="1">Kategorie 1</option>
<option value="2">Kategorie 2</option>
<option value="3">Kategorie 3</option>
</select>
</p>
<p><input type="hidden" name="form_id" value="test" /><input type="submit" /></p>
</form>
</body>
</html>
